Mobile communication such as Long Term Evolution (LTE) and LTE-advanced are conventionally known (for example, refer to 3GPP TS36.300 V12.1.0, March 2014; 3GPP TS36.211 V12.1.0, March 2014; 3GPP TS36.212 V12.0.0, December 2013; 3GPP TS36.213 V12.1.0, March 2014; 3GPP TS36.321 V12.0.0, December 2013; 3GPP TS36.322 V11.0.0, September 2012; 3GPP TS36.323 V11.2.0, March 2013; 3GPP TS36.331 V12.0.0, December 2013; 3GPP TS36.413 V12.0.0, December 2013; 3GPP TS36.423 V12.0.0, December 2013; 3GPP TR36.842 V12.0.0, December 2013; and 3GPP TS24.301 v12.6.0, September 2014). Further, Proximity-based Services (ProSe) (function of direct communication between terminals) enabling direct communication between terminals is being studied (for example, refer to 3GPP TR36.843 V12.0.0, March 2014; and 3GPP TS23.303 v12.0.0, February 2014).
Isolated E-UTRAN Operation for Public Safety (IOPS) enabling continuous communication even when a wireless access network is isolated from a packet core network is being studied (for example, refer to 3GPP TR22.897 v13.0.0, June 2014). A packet core network is, for example, an Evolved Packet Core (EPC).
Further, according to a known a technique, when a femto base station or a gateway device detects transmission failure with a core network, instead of the core network, call connection is established between mobile stations within the cellular coverage of the femto base station (for example, refer to Japanese Laid-Open Patent Publication No. 2010-233184).
According to another known technique, when a communication line failure between a base station and a higher device occurs, the base station changes the simultaneous call area number of the base station ID to be broadcast and causes communication terminals in the cell of the base station to perform position registration (for example, refer to Japanese Laid-Open Patent Publication No. 2013-131788).
According to another known technique, when a switch is congested and the switch denies a terminal's request for communication with another terminal, direct communication between the terminals is permitted (for example, refer to Japanese Laid-Open Patent Publication No. 2013-223194).
According to another known technique, when a communication line connection between a base station and a control station is lost, the base station terminates the transfer of a mobile-originated call signal to the control station from a mobile station of the call originating side and transmits the call signal to a mobile station of the call receiving side, allowing the mobile station to receive the call (for example, refer to Japanese Laid-Open Patent Publication No. H10-304425).